1. Field of the Invention
This invention relates to a flexible sheet dam used an an intake weir for catching agricultural water or the like.
2. Related Art Statement
Heretofore, a collapsible rubber dam, employed a flexible rubber bag attached onto a riverbed and both riverbank portions to cross a river and a supply. A discharge means was arranged on the riverbank portion, and this structure has commonly been used. In this case, a fluid such as air or water was supplied into the inside of the rubber bag through the supply and discharge means to inflate the bag, while the fluid filled in the bag was discharged through the supply and discharge means to deflate the bag.
As a means for attaching the rubber bag to the river, a one-side fixing system is known in which only one side edge of the rubber bag was fixed to the riverbed and riverbank portions in an upstream side, and a two-side fixing system in which both side edges of the rubber bag were fixed at a given interval in upstream and downstream sides used as a tide weir.
According to the conventional technique, the above one-side fixing system is used when the water depth at the downstream side of the dam is usually low in general rivers or the like. In this case, there is caused no serious problem as much as the dam is used at such a state. However, when the river width becomes narrow toward the downstream side, or when plural dam gates are successively deflated, if the water depth at the downstream side of the dam increases at the deflation stage of the rubber bag, water pressure is applied to the rubber bag from the downstream side to create a force of raising the rubber bag, and consequently the rubber bag is elongated to cause cramping of a portion fixing the supply and discharge tube and a top of an upper portion fixed onto each riverbank.
In case of the two-side fixing system, the rubber bag is fixed at the upstream side and downstream side, so that the force of raising the rubber bag can be controlled to a certain extent to prevent the above cramping. In this case, however, the production cost of the dam structure is increased due to the increase in the number of fitting members and the working step number. Furthermore, when the flow rate is particularly slow or the water depth at the downstream side increases during the deflation of the rubber bag, as shown in FIGS. 9a to 9e, a portion of the rubber bag facing the downstream side is bent between the two fixed positions and the deflated shape of the rubber bag becomes higher at the downstream side, so that the sectional shape of water flowing over the dam is crashed to badly affect the flowing in the river. Furthermore, the formation of the bent portion in the rubber bag brings about damage of the bag.